The motors in vacuum cleaners, hand-held power tools, and other small electrically-powered devices include brushes that engage a commutator on a central rotor.
Examples of designs for mounting the brushes of electric motors in general can be found in Nakata et al, U.S. Pat. No. 4,990,811; Bulick, U.S. Pat. No. 4,868,441; Gotoh, U.S. Pat. No. 4,554,476; and Iwaki et al, U.S. Pat. No. 4,504,752. Designs specifically for use in small electric motors can be found in Wacek et al, U.S. Pat. No. 4,800,312 and in Hall et al, U.S. Pat. No. 3,430,084.
It is common to use a helical compression spring to bias a brush against a commutator. The spring is mounted at an outer end of the brush with the axis of the spring aligned with the length of the brush. The spring extends as friction between the brush and the rotating commutator wears down the inner end of the brush, maintaining contact between the brush and the commutator.
One disadvantage of such designs results from the fact that the force of the spring decreases as the spring extends. This decrease in force causes the pressure between the commutator and the brush to continually decrease as the brush wears down. This eventually leads to poor contact between the brush and commutator, and poor motor performance.
There have been attempts to replace helical compression springs with coiled constant force springs (sometimes referred to as Hunter's springs) in order to obtain a constant force between the brush and the commutator throughout the life of the brush. Bulick and Nakata et al both use a spring assembly involving two coiled constant force springs mounted on opposite sides of a brush. A flat piece connects the ends of the two springs and presses against the rear of the brush.
One disadvantage of these designs appears to be increased cost and complexity of assembly.